1. Field of the Invention
The present invention relates to the field of medical devices, more specifically to the field of implantable coronary stents.
2. Description of Related Art
As is known, the human heart circulates blood throughout the body. Depending on the individual, the heart beats between 80,000 and 140,000 times per day. During normal function of the heart, the left and right atria and the left and right ventricles contract, causing blood to flow. The blood flows from the heart, passes through a set of blood vessels known as arteries that feed the organs and tissue in the body and then returns to the heart through a set of blood vessels known as veins. This circulation provides nutrients and oxygen to the body so that it can continue to function.
As the heart is basically a continuously functioning muscle, it also needs a steady supply of nutrients in order to function. For example, a coronary artery supplies blood (and the associated oxygen and nutrients) to the cardiac muscle. In order for the heart to continue to function, it is crucial that this artery continue to function properly.
Unfortunately, the coronary artery can become partially or completely blocked. One cause is left main coronary artery disease (“LMCD”). LMCD may be caused, for example, by the accumulation of fatty tissue on the wall of the left main artery. LMCD is generally defined as a greater than 50% reduction in the left main, which results in insufficient blood flow to the heart tissue and eventually causes damage to the heart tissue.
While partial blockage can result in permanent damage to the heart muscle, sudden complete blockage of the left main will result in the death of the individual. Therefore, maintaining blood flow through the left main is crucial to an individual's ability to exist. Any symptomatic blockage must be immediately treated.
Two methods of treatment of a partially or completely blocked left main are 1) percutaneous transluminal coronary angioplasty (“PTCA”), also referred to as percutaneous coronary intervention (“PCI”), and commonly referred to as balloon angioplasty or angioplasty, and 2) coronary artery bypass graft (“CABG”), commonly referred to as bypass surgery. Due to a number of factors, the most common procedure to treat LMCD has been bypass surgery.
In essence, bypass surgery uses section of veins or arteries sections from other parts of the body to connect the aorta to a point downstream of the blockage. This allows blood to flow around the blockage point through a separate passageway. Depending on the severity and location of the blockage, as many as four to five grafts are necessary.
One downside to bypass surgery is that it takes a substantial time to perform. For a patient with a totally occlusion or a severely blocked left main, the time it takes to perform bypass surgery may be too long. Therefore, in emergencies, balloon angioplasty has been performed on patients suffering from sudden LMCD.
Another problem is that bypass surgery is only effective for about 8-10 years, at which point a patient generally requires additional treatment that is generally less effective. Given the potential long-term health problems, it is desirable to delay bypass surgery if possible.
Furthermore, certain patients' medical conditions are incompatible with the rigors of bypass surgery. For example, some patients have severe co-morbid conditions precluding open-heart surgery, such as malignancy with limited life-expectancy, no longer are a candidate for bypass surgery. Thus, while bypass surgery is a useful medical procedure that has saved many lives, it is best saved for situations where less complex procedures cannot be used effectively.
In addition, some interventional cardiac catheterization labs are not backed up by surgical programs. This is problematic in situations where the LMCD must be treated immediately (e.g. iatrogenic dissection of the left main).
Compared to bypass surgery, angioplasty can be done relatively quickly and is generally less traumatic to the patient. Basically, during angioplasty a wire is inserted into the artery. A flexible catheter is then guided along the wire. A balloon attached to the catheter is positioned in the left main at the point of blockage and the balloon is inflated to open the artery. To keep the artery open, a stent may be placed in the left main. One common method of delivering the stent is to wrap it around the balloon. Thus, the inflation of the balloon causes the stent to expand into position. The stent acts as a scaffolding to support the wall of the artery and, when coated with anti-restenotic agents, can be an alternative means of treating certain types of LMCD.
While the expansion of blocked portions of arteries with angioplasty can be effective, certain areas of the left main have proven difficult to treat with balloon angioplasty. Sometimes the blockage occurs at a point of junction between the left main and the left anterior descending artery and the left circumflex artery. In such a situation, expansion and insertion of a stent into one of the secondary branches has the tendency to jeopardize the other branch.
Attempts have been made to provide a customized stent that can be used in a junction to support the main branch and the two secondary branches but such an approach has limitations. For one thing, the secondary branches and angles between the different branches are varied from person to person as well as from junction to junction, making it difficult to design a single stent that can work with all the potential variations. Furthermore, such a design is difficult to install, thus making it less attractive to locations such as the junction in the left main. Another major limitation is that much higher operator skill is required to position such a stent quickly, thus making it more likely that such a stent will be improperly installed.
Therefore, often the only choice is to attempt bypass surgery. However, in the face of complete blockage it is possible that a patient will not survive the procedure. Clearly, something more is needed to address such a life-threatening emergency. In addition, it would be beneficial to provide a stent that could be used in and near the left main junction during more routine medical situations as an alternative to bypass surgery.